"Everything" sung by Anna Tsuchiya and Olivia, who provided inspired songs that were used in the anime (as OP themes, ED themes, and insert songs) for their respective counterparts (punk-styled rock for Nana and the Black Stones, and alternative rock for Reira and Trapnest), such as Tsuchiya's "Kuroi Namida" and Olivia's "Wish".
The insanely infectious "Glamorous Sky" (sung by Mika Nakashima) from the movie.
The main plot of the series is about the relationship about both Nanas, and how it develops over time. However, as they separate, the audience realizes that every single person and situation around them turn out to be shittier than they originally were. With the manga on hiatus since 2009, the last meeting between both of them raised uncertainty about their future as friends (which would probably lead to a final meeting before the "Present Time" takes place).
Hachi's Will They or Won't They? relationship with Nobu was very close to be extremely tiring, regardless of the advancements between those two. Too bad the whole mess about her pregnancy brought Takumi back into her life, and some chapters later, she loudly dismissed him over the phone after overhearing him complaining about how pregnant women are such a bother.
While Nana K. is well-loved for being realistic, she also gets criticism for being a Naïve Everygirl and, especially, her decision to choose Takumi over Nobu when she gets pregnant.
Junko may act like the Greek Chorus and shares the same frustrations the audience has with Nana K., but she can be Brutally Honest and harsh on her as well. The fanbase has particularly criticized her for how she handled Nana K. and Shoji's break-up, as her comments about how Nana K. didn't "love" Shoji as much as Sachiko did, seemed on par with Victim-Blaming. She also lectures Nana K. and tells her that it's her fault that she got pregnant, as she never demanded Takumi use condoms.
While Sachiko did get in between girlfriend and boyfriend, it still sucks that she's the one who gets almost all the shit, even though it was clearly Shoji who was two-timing with her and Hachi.
Reira receives a lot of hate from both Takumi/Hachi shippers AND Nana/Ren shippers. Takumi, however, is still loved by fans even when he is cheating on Hachi without a second thought, while his feelings towards her settle only slightly above indifference; and of course, Ren is the one who has been shown to have previous feelings for Reira, while Reira sees him more as a confidant and a friend, and was clearly upset when the press threatened to release pictures that would persuade the public they were having an affair. Perhaps a milder version of Ron the Death Eater.
However, throughout the years, the hate towards Takumi has started to flock up with both Hachi/Nobu and Hachi/Nana shippers beating on him as a horrible asshole who should just leave Hachi alone so she can be with someone who'll treat her well. Although unlike most examples, the hate towards Takumi is definitely not as unwarranted considering he really isn't a pleasant person and did make things more complicated.
Takumi. Big time. Interestingly enough, this is a more recent sentiment - back then, hate towards Takumi wasn't as vocal, but today, most Nana fans, even old ones, utterly despise him and wished he would be written out of the story and finally leave Hachi alone. For valid reasons, or for shipping reasons.
Reira is not well-liked by the audience for several reasons. One of the biggest reasons, at least in the West, is because of her relationship with Shin, which is seen as a source of Squick. Despite that Reira is pretty naive and immature to the point that Shin is more mature than she is, they were 22 and 15 years old respectively when they first start having sex. In the 2010s, age-gap relationships and power imbalances began to be more heavily scrutinized, and as a result, Reira is seen as a predator because of her age and higher social standing. Furthermore, after they break up, Reira immediately tries to pursue a relationship with Takumi despite knowing he's married and also seeks Ren for help, causing him to devote his time to her and make the rift between him and Nana O. wider.
Takumi may have made some contentious decisions, but he's rational and right about a lot of things.
After Nana O. and Ren's relationship gets leaked to the press, Nana K.'s first instinct is to go to room 707 to protect Nana O. Takumi refuses to let her, and the two get into an argument. When she calls Junko and Kyosuke to vent about this, Kyosuke calmly tells her that Takumi's decision is the better one, as Nana K. could accidentally cause more unwanted media attention due to being both Nana O.'s roommate and Takumi's fiancee.
Junko discussing why Nana K. and Shoji's relationship fell apart has been a sort of contention within the fandom, because she comes off as Victim-Blaming for Nana K and refuses to take a side, even though Shoji was clearly in the wrong. However, she is right that there were many problems in Nana K. and Shoji's relationship already that a break-up was pretty much inevitable, whether or not he had cheated.
Wangst: Sometimes, Hachi acknowledges that she complains a lot even when she shouldn't.
What Do You Mean, It's for Kids?: The manga is meant to be shoujo, but it's often been mistaken for josei due to most of the characters being young adults and its depiction of romance being more realistic and grounded than most manga aimed at teenage girls.
Why Would Anyone Take Him Back?: As the years went by, a very common sentiment were fans wondering why Hachi took Takumi back.
